1. Field of the Invention
The present invention relates generally to variable voltage protection devices used to protect electronic circuits from overvoltage transients caused by lightning, electromagnetic pulses, electrostatic discharges, ground loop induced transients, or inductive power surges.
2. Description of Related Art
It is generally known that the magnitude of an electrical voltage allowed to be applied to an integrated circuit package is rather limited since the physical size of the integrated circuit package is fairly small. When the integrated circuit package is not being used, for example, in storage or handling, the external leads or pins thereof are susceptible to the build-up of a static charge thereon. If the integrated circuit package happens to come in contact with a ground potential, the accumulated static charges will flow to ground. Such static discharge can be of a catastrophic nature with sufficient energy to cause damage or even destroy the semiconductor element or chip mounted within the integrated circuit package.
In order to protect the semiconductor chip in the integrated circuit package from being destroyed when such static discharges or other overvoltages occur, there has been provided in the prior art a protection element such as a transistor element or a p-n junction applied with a reverse bias, which breaks down when the semiconductor chip encounters an unexpectedly high voltage.
Other protection schemes, such as that disclosed in U.S. Pat. No. 4,928,199 to Diaz, et al., use a circuit protection device incorporated into an IC package. In the Diaz, et al. patent the device, referred to as a threshold circuit protection device, is disposed against the conductive pads of the IC package and operates to ground excess current resulting from potentially damaging voltage transients encountered by the IC-package combination. The Diaz, et al. device itself is annular in shape and comprised of a glass layer and a conducting layer connected to ground. Voltages exceeding a threshold level cause the glass layer to conduct, thereby providing a direct pathway for current away from the sensitive circuitry of the IC. The glass layer operates as an insulator under normal voltages below the threshold voltage level.
In U.S. Pat. No. 5,796,570 to Mekdhanasarn, et al., a circuit board is provided with protection against electrostatic discharge. The circuit board includes a plurality of interconnect traces and an electrically conductive ground plane separated from each other by a gap containing a resistive discharge protection material. The material serves to insulate the interconnect traces from the ground plane during normal operation, but becomes conductive in the presence of voltages above a threshold voltage.
Various overvoltage protection materials have been used in the prior art, such as the glass material used in the Diaz, et al. patent. These materials are also known as nonlinear resistance materials and are herein referred to as variable voltage materials. In operation, the variable voltage material initially has high electrical resistance. When the circuit experiences an overvoltage spike, the variable voltage material quickly changes to a low electrical resistance state in order to short the overvoltage to a ground. After the overvoltage has passed, the material immediately reverts back to a high electrical resistance state. The key operational parameters of the variable voltage material are the response time, the clamp voltage, the voltage peak and peak power. The time it takes for the variable voltage material to switch from insulating to conducting is the response time. The voltage at which the variable voltage material limits the voltage surge is called the clamp voltage. In other words, after the material switches to conducting, the material ensures that the integrated circuit chip, for example, will not be subjected to a voltage greater than the clamp voltage. The voltage at which the variable voltage material will switch (under surge conditions) from insulating to conducting is the switch voltage. These materials typically comprise finely divided conductive or semiconductive particles dispersed in an organic resin or other insulating medium. For example, U.S. Pat. No. 3,685,026 (Wakabayashi, et al.), U.S. Pat. No. 4,977,357 (Shrier) and U.S. Pat. No. 4,726,991 (Hyatt et al.) disclose such materials.
Variable voltage materials and components containing variable voltage materials have been incorporated into overvoltage protection devices in a number of ways, including that discussed in U.S. Pat. No. 4,928,199 (Diaz, et al.). Other examples include U.S. Pat. Nos. 5,142,263 and 5,189,387 (both issued to Childers et al.), which disclose a surface mount device which includes a pair of conductive sheets and variable voltage material disposed between the pair of conductive sheets. U.S. Pat. No. 5,246,388 (Collins et al.) is directed to a device having a first set of electrical contacts that interconnect with signal contacts of an electrical connector, a second set of contacts that connect to a ground, and a rigid plastic housing holding the first and second set of contacts so that there is a precise spacing gap to be filled with the overvoltage material. U.S. Pat. No. 5,248,517 (Shrier et al.) discloses painting or printing the variable voltage material onto a substrate so that conformal coating with variable voltage material of large areas and intricate surfaces can be achieved. By directly printing the variable voltage material onto a substrate, the variable voltage material functions as a discrete device or as part of associated circuitry. Finally, U.S. Pat. No. 5,796,570 (Mekdhanasarn, et al.) discloses a circuit board having a plurality of interconnect traces and an electrically conductive ground plane formed on a substrate such that a gap is created between the interconnect traces and the ground plane. The gap contains a resistive electrostatic discharge protection material which insulates the interconnect traces from the ground plane at voltages below a predefined threshold voltage and establishes an electrical connection between the interconnect traces and the conductive plane at voltages above the threshold voltage.
Although the prior art discloses various materials and devices, there is a continuing and long felt need to provide improved cost-effective variable voltage materials and devices. Moreover, as miniaturization of semiconductor integrated circuits increases, the amount of circuit components comprising individual chips has dramatically increased. Accordingly, the number of connections which need to be made with an electronic package housing such a high density of circuit components has increased. The need to protect the dense integrated circuits from overvoltages occurring at any and all of these connections presents unique problems associated with the physical layout of the electronic packages, problems which are not addressed by the prior art. These same problems of physical spacing and materials selection, for example, also figure in the design of the printed circuit boards upon which the IC packages arc intended to mount.
The invention provides an arrangement for protecting sensitive electronic devices such as integrated circuits from voltage transients. In accordance with the invention, the package in which an integrated circuit (IC) is housed is provided with ground bars each adapted for connection to ground and each associated with one or more contact portions serving to electrically connect the IC package (and the IC housed therein) to a confronting exterior circuit such as one disposed on a printed circuit board (PCB). The package itself may house one or more IC chips and may be of the form of a ball grid array (BGA), a pin grid array (PGA), a dual inline package (DIP), a single inline package (SIP) or any similar integrated circuit packaging structure.
The association between the ground bar and the contact portions is in the form of a microgap formed between the ground bar and each contact portion, with the microgap containing therein a variable voltage material, preferably the proprietary material known as SurgX(trademark). The microgap with the variable voltage material provides for electrical communication between the contact portion and the ground bar under controlled conditions, namely when a predetermined threshold voltage is exceeded. Below the threshold, the microgap behaves as an insulator, whereas when a voltage spike due to for example static discharge is presented thereto, a clamping voltage barrier is overcome and the microgap-variable voltage combination behaves as a very low resistance conductor serving to short the contact portion to ground, thereby safely dissipating any excess current.